Power device with bottom source electrode

ABSTRACT

A power semiconductor package has an ultra thin chip with front side molding to reduce substrate resistance; a lead frame unit with grooves located on both side leads provides precise positioning for connecting numerous bridge-shaped metal clips to the front side of the side leads. The bridge-shaped metal clips are provided with bridge structure and half or fully etched through holes for relieving superfluous solder during manufacturing process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Patent Application is a Continuation in Part (CIP) Application of aco-pending application Ser. No. 13/289,918 filed on Nov. 4, 2011 by acommon inventor of this Application. Application Ser. No. 13/289,918 isa Divisional Application and claims the Priority Date of anotherapplication Ser. No. 12/606,290 filed on Oct. 27, 2009 now issued intoU.S. Pat. No. 8,076,183. This Patent Application is also a Continuationin Part (CIP) Application of a co-pending application Ser. No 12/968,159filed on Dec. 14, 2010 by a common inventor of this Application. TheDisclosures made in the patent applications Ser. Nos. 13/289,918,12/606,290, and 12/968,159 are hereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a semiconductor power device and preparationmethod thereof. Particularly, this invention aims at providing a powerdevice with a bottom source electrode and preparation method thereof.

DESCRIPTION OF THE RELATED ART

Power consumption of power devices is commonly very high. In theapplication of DC-DC power converter devices, some metal electrodes ofthe devices are usually exposed from plastic packaging material coatinga semiconductor chip for improving the electrical connection and heatdissipation performance of the devices. For example, as shown in FIG. 1,US patent application publication US2003/0132531A1 discloses asemiconductor packaging structure 24 with a bottom electrode of asemiconductor chip exposed and used for supporting surface mountingtechnology. Here, a power MOSFET 10 is arranged in an interior space ofa cup-shaped metal can 12 and a drain electrode at one side of theMOSFET 10 is connected to the bottom of the interior space of thecup-shaped metal can 12 through a layer of conductive epoxy 14, so thatthe drain electrode of the MOSFET 10 is electrically connected to anextruding edge 22 of the cup-shaped metal can 12, while a sourceelectrode 18 and a gate electrode (not shown) located at the other sideof the MOSFET 10 become sub-flush with the surface of the extruding edge22. Low stress and high adhesion epoxy 16 is provided to fill in gaps inthe interior space of the cup-shaped metal can 12 surrounding the MOSFET10. The semiconductor packaging structure 24 improves the heatdissipation performance. However, it is expensive to form the cup-shapedmetal can 12 in actual production. In addition, both the sourceelectrode and the gate electrode of the MOSFET 10 are fixed in thepackaging structure 24, as a result the contact surface of the gateelectrode cannot be adjusted to level with the extruding edge 22, thusit is hard to match the contact surface of the gate electrode with a padon a PCB (Printed Circuit Board), which limits the application of thesemiconductor packaging structure 24.

In addition, the resistance of a substrate in the chip of the powerdevice is usually high, this makes the RDSon of the devicecorrespondingly high; therefore, there is a need to reduce theresistance of the substrate of the chip. In a conventional wafer levelchip scale packaging (WLCSP), packaging test is performed and ballplacement on a wafer (for ball bonding) is carried out after theprocessing of all power devices in the whole wafer is completelyfinished, individual IC (Integrated Circuit) is then singulated with itssize being same as the desired original chip.

Given the above description of related prior arts, therefore, there is aneed to manufacture ultra thin chips by WLCSP and to apply these chipsin power devices.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiment of the present invention is described more sufficientlythrough the drawings. However, the drawings are only used for explainingand illustrating rather than limiting the scope of the invention.

FIG. 1 is a cross sectional schematic diagram of the semiconductorpackaging structure of the prior art.

FIGS. 2A-2E are structural schematic diagrams of the power devicesaccording to a first embodiment of the present invention.

FIGS. 3A-3F are schematic diagrams illustrating a process for preparingthe primary packaging structure of the power devices of the presentinvention.

FIGS. 4A-4C are cross sectional schematic diagrams illustrating aprocess for preparing the power devices of the present invention.

FIGS. 5A-5B are cross sectional schematic diagrams illustrating thepower devices according to a second embodiment of the present invention.

FIGS. 6A-6D are structural schematic diagrams illustrating the powerdevices according to a third embodiment of the present invention.

FIGS. 7A-7C are structural schematic diagrams illustrating the powerdevices according to a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2A and FIG. 2B are schematic diagrams showing a top view and abottom view of the power device 100A of a first embodiment of thepresent invention. FIG. 2C and FIG. 2D-1 are cross sectional views ofthe power device 100A along a plane AA and along a plane BB respectivelyshown in FIG. 2B. The power device 100A comprises a lead frame unitincluding a first base 111, a second base 112, a third base 113 and afourth base 114 (as shown in FIG. 2B). The thicknesses of all the basesare basically the same and the bases are arranged on the same plane. Thefirst base 111 preferably has a square shape. The third base 113 and thefourth base 114 are arranged at two opposite sides of the first base 111respectively and extend along the edges of the first base 111, and thesecond base 112 is positioned adjacent to the first base 111 between thethird base 113 and the fourth base 114. In one embodiment, the thirdbase 113 and the fourth base 114 are symmetrical relative to the centerof the first base 111, thus the second base 112 is positioned on a lineof symmetry of the third base 113 and the fourth base 114. Usually, alead frame strip comprises a plurality of such lead frame units with thebases connected to the lead frame strip through connecting bars (notshown).

As shown in FIG. 2C and FIG. 2D-1, the power device 100A also comprisesa primary packaging structure 130 that is a completely packagedstructure rather than an unpackaged original chip. The primary packagingstructure 130 includes a semiconductor chip 131 that is flipped andattached onto the first base 111 and the second base 112. Conductiveepoxy (such as conductive solder or paste) is used for forming aplurality of balls or bumps 132 a-1 and 132 b-1 on the front surface ofthe primary packaging structure 130 for attaching it onto the secondbase 112 and the first base 111 respectively. As shown in FIGS. 2C and2D-1, the primary packaging structure 130 comprises a semiconductor chip131 and a top plastic packaging layer 134 covering the front surface ofthe chip 131. The front surface of the chip 131 is provided with aplurality of metal pads. The solder bumps 132 a-1 and 132 b-1 are placedon the metal pads, which will be described in details later. The topplastic packaging layer 134 of the primary packaging structure 130 areonly encapsulated on the side walls of the solder bumps 132 a-1 and 132b-1. A bottom metal layer 133 is formed at the back surface of the chip131.

The power device 100A also comprises a bridge-shaped metal clip 150(also shown in FIG. 2E) attached to the bottom surface of the flippedprimary packaging structure 130 or to the bottom surface of the bottommetal layer 133 of the flipped chip 131. The bridge-shaped metal clip150 is also attached to the third base 113 and the fourth base 114. Thebridge-shaped metal clip 150 comprises a top metal portion 151 and sidemetal portions 153 a and 153 b connected to two opposite sides of thetop metal portion 151. The side metal portions 153 a and 153 b are bentdownwards as shown in FIG. 2C. In particular, the side metal portions153 a and 153 b are bent away from each other, so that the angles formedbetween the side metal portions 153 a and 153 b and the top metalportion 151 are obtuse angles. In one embodiment, the side metalportions 153 a and 153 b are symmetrically located relative to thecenter of the top metal portion 151. In addition, a groove 113 a isformed on the top surface of the third base 113 and a groove 114 a isformed on the top surface of the fourth base 114. As a result the sidemetal portion 153 a can be attached and trapped in the groove 113 a,likewise the side metal portion 153 b can be attached and trapped in thegroove 114 a. A conductive material 140, such as a conductive solder orpaste, is applied to bond the bottom surface of the top metal portion151 to the bottom metal layer 133. The groove 113 a and the groove 114 acan be of many shapes, for example, the grooves 113 a and 114 a can bein V-shape, as shown in FIG. 2C, for convenient engagement with the sidemetal portions 153 a and 153 b. The side metal portions 153 a and 153 bare respectively attached to the third base 113 and the fourth base 114through conductive epoxy deposited in the groove 113 a and the groove113 b.

Furthermore, the power device 100A also comprises a plastic packagingbody 160 encapsulating the lead frame unit, the primary packagingstructure 130 and the bridge-shaped metal clip 150. As shown in FIG. 2Bwhere the power device 100A is finally mounted on a PCB with the firstbase 111, the second base 112, the third base 113 and the fourth base114 serving as electrical contacts directly connected to the pads on thePCB. Here, the respective bottom surfaces of the first base 111, thesecond base 112, the third base 113 and the fourth base 114 should beexposed from the bottom surface of the plastic packaging body 160.Furthermore, as shown in FIG. 2B, the third base 113 and the fourth base114 usually include a plurality of pins, for example pins 113′ and pins114′. Therefore, the bottom surfaces of the pins 113′ and the pins 114′are also exposed from the bottom surface of the plastic packaging body160 and serving as the electrical contacts of the third base 113 and thefourth base 114.

In the embodiment shown in FIGS. 2C to 2D-1, one or more through holes152 are formed through the whole thickness of the top metal portion 151.FIG. 2E shows a top view of the bridge-shaped metal clip 150 including athrough hole 152. The through hole 152 can be of a ‘cross’ shape asshown in FIG. 2E or can be of round, rectangle, polygon or any othersuitable shapes. The through hole 152 is used for venting gas during thereflow of the conductive material 140 that attaches the top metalportion 151 to the bottom metal layer 133. Additionally, any excess ofthe conductive material 140 deposited to form a conductive layer betweenthe top metal portion 151 and the bottom metal layer 133 can be dredgedinto the through holes 152 so that the final layer thickness of theconductive material 140 is uniform.

In another embodiment as shown in FIG. 2D-2, one or more clip grooves152′ can be formed from a bottom surface of the top metal portion 151with the bottom of the clip groove 152′ ended up inside the top metalportion 151. The groove 152′ may be of many shapes similar to thethrough holes 152 as described above. Similar to the though hole 152,the clip groove 152′ is used for venting gas during the reflow of theconductive material 140 for attaching the top portion 151 to the bottommetal layer 133 and for holding any excess of the conductive material140 deposited to form a conductive layer between the top metal portion151 and the bottom metal layer 133 thus improving the thicknessuniformity of the conductive material 140.

Another difference between the embodiment of FIG. 2D-1 and that of FIG.2D-2 is that the top surface of the top metal portion 151 as shown inFIG. 2D-1 is not exposed from the plastic packaging body 160 whereas thetop surface of the top metal sheet 151 as shown in FIG. 2D-2 is exposedfrom the plastic packaging body 160. To achieve a structure as shown inFIG. 2D-2, before depositing a plastic packaging material, such as epoxyresin, to form the plastic packaging body 160, a resist film (not shown)can be applied to the inner surface of the top chase of the moldingtool, which is then brought in contact with and covers the top surfaceof the top metal portion 151 thus preventing it from coverage by theplastic packaging materials. The plastic packaging body 160 encapsulatesthe lead frame unit, the primary packaging structure 130 and the sidemetal portions 153 a and 153 b of the bridge-shaped metal clip 150.After the plastic packaging material is solidified, the resist film ispeeled off from the top surface of the top metal portion 151, thus thetop surface of the top metal portion 151 is now exposed from the topsurface of the plastic packaging body 160. This plastic packagingprocess is usually completed at a wafer processing level (i.e., thistechnology is used in WLCSP), which is well known in the art.

In another embodiment of the invention, recessed portions 154 a and 154b formed on the top surface of the top metal portion 151 at the cornerof the top metal portion 151 are configured to connect the side metalportions 153 a and 153 b thus forming a step structure. FIGS. 2C-2Eillustrate the structure of the bridge-shaped metal clip 150. Typically,the side metal portions 153 a and 153 b are originally formed on thesame plane of the top metal portion 151, then the side metal portions153 a and 153 b are bent downward by an angle (through a stampingmethod), so that the angles formed between the side metal portions 153 aand 153 b and the top metal portion 151 are obtuse angles. However, thethus obtained final top metal portion 151 is not a flat plane and theedges of the top surface of the top metal portion 151 at the corner ofthe top metal portion 151 and the side metal portions 153 a 153 b do notform a straight line. Therefore, the recessed portions 154 a and 154 bat the corner of the top metal portion 151 and the side metal portions153 a 153 b can beneficially buffer and stop the tension influences ofthe side metal portions 153 a and 153 b on the top metal portion 151during the stamping step with the thus obtained top metal portion 151free of deformation, in which case lines 151 a-1 and 151 a-2 at the twosides of the top surface of the top metal portion 151 are now straightlines and the top surface of the top metal portion 151 is now a flatrectangular plane.

FIGS. 3A-3F illustrate a method for preparing the primary packagingstructure 130. A wafer 1310 (shown in FIG. 3C) usually includes numeroussemiconductor chips 131 (shown in FIG. 3A) formed at the top surface ofthe wafer and spaced-apart by scribe lines (not shown), which is wellknown in the art. The front surface of the chip 131 includes numerousmetal pads 132, such as aluminum-silicon pads, which serve as theelectrodes of the chip or the terminals for off-chip signaltransmission. In a preferred embodiment, the chip 131 is a verticalpower metal oxide semiconductor field effect transistor (MOSFET). Themetal pads 132 include metal pads 132 b forming the first electrode(such as a source electrode) of the chip 131 and a metal pad 132 aforming the second electrode (such as a gate electrode) of the chip 131,while the drain electrode area of the chip 131 is formed at the backsurface of the chip 131 (not shown). Firstly, numerous solder bumps areformed on the metal pads 132 by ball placement or plating and the likes.As shown in FIG. 3B, a solder bump 132 a-1 is formed on the metal pad132 a and a solder bumps 132 b-1 are formed on the metal pads 132 b. Asthe area of the metal pad 132 b forming the source electrode is usuallylarger than that of the metal pad 132 a forming the gate electrode, thesize of the solder bumps 132 b-1 is also larger than that of the solderbumps 132 a-1 to carry large currents. Alternatively, numerous solderballs of smaller size than the solder bump 132 b-1 can be placed on themetal pad 132 b (not shown) and are closer to each other, so that thesolder balls can be merged into one piece after being heated, softenedand melted to form the solder bump 132 b-1 of a larger size. As shown inFIG. 3C, a plastic packaging layer 1340 is formed on the front surfaceof the wafer 1310 covering all the solder bumps 132 a-1 and 132 b-1.Then the plastic packaging layer 1340 is ground until the solder bumps132 a-1 and 132 b-1 are exposed through the plastic packaging layer1340. As shown in FIG. 3D, the top surfaces of the solder bumps 132 a-1and 132 b-1 and the top surface of the plastic packaging layer 1340 areco-planar. The plastic packaging layer 1340 physically supports thewafer 1310. Therefore, when the wafer 1310 is ground and thinned, thewafer 1310 is not prone to crackage. This means that highly desirableultra-thin chips with reduced substrate resistance can be made. As shownin FIG. 3E, after the back surface of the wafer 1310 is ground andthinned, impurity ions can be heavily doped into the back surface of thethinned wafer 1310 (optionally), and then a metal layer 1330 can bedeposited onto the back surface of the thinned wafer 1310 forming thedrain electrode at the back surface of the chip. The wafer 1310, theplastic packaging layer 1340 and the metal layer 1330 (as shown in FIG.3E) are then cut apart to form individual primary packaging structures130 (as shown in FIG. 3F), each of which includes a single chip 131 anda top plastic packaging layer 134 covering the front surface of the chip131. The top plastic packaging layer 134 only covers the side walls ofthe solder bumps 132 a-1 and 132 b-1 with the top surface of the solderbumps 132 a-1 and 132 b-1 exposed through the top plastic packaginglayer 134 and is co-planar with the top surface of the top plasticpackaging layer 134. In this step, the metal layer 1330 is also cutapart into numerous bottom metal layers 133, each of which covers theback surface of a chip 131 and is contact with the drain area at theback surface of the chip 131 forming the third electrode (such as thedrain electrode) of the chip 131.

As shown in FIG. 2D-1, the solder bump 132 b-1, formed on the metal pad132 b forming the first electrode of the chip, is attached to the topsurface of the first base 111. As shown in FIG. 2C, the solder bump 132a-1, formed on the metal pad 132 a forming the second electrode of thechip, is attached to the top surface of the second base 112. As shown inFIG. 2B, the surface area of the first base 111 forming the sourceelectrode is usually larger than the surface area of the second base 112forming the gate electrode. Therefore, the exposed area of the bottomsurface of the first base 111 is larger than the exposed area of thebottom surface of the second base 112, which also performs the functionof heat dissipation. The third base 113 and the fourth base 114 areelectrically connected to the drain electrode of the chip 131 throughthe bridge-shaped metal clip 150.

FIGS. 4A-4C illustrate a method for preparing the power device 100Ashown in FIG. 2D-1 along the line BB of FIG. 2B. However, thepreparation of the power device 100A shown in FIG. 2C along the line AAof FIG. 2B is also described but not shown in FIGS. 4A-4C. In FIG. 4A, alead frame unit is provided firstly. The lead frame unit includes thefirst base 111, the second base 112, the third base 113 and the fourthbase 114, all of which are separated from each other, with the thirdbase 113 and the fourth base 114 respectively arranged at the twoopposite sides of the first base 111 as described above. The primarypackaging structure 130 is then attached on the first base 111 and thesecond base 112 of the lead frame unit by a conductive epoxy. In thisstep, the plurality of solder bumps 132 b-1 and 132 a-1 (see FIG. 3F)formed on the front surface of the primary packaging structure 130 arerespectively attached to the first base 111 and the second base 112 by aconducting material, such as the conducting material 120 b shown in FIG.4A. In FIG. 4B, the bridge-shaped metal clip 150 is mounted atop theprimary packaging structure 130. The bridge-shaped metal clip 150comprises the top metal portion 151 and the side metal portions 153 aand 153 b connected to two opposite sides of the top metal portion 151and bent downwards. In this step, the top metal portion 151 is directlyattached to the primary packaging structure 130. The side metal portions153 a and 153 b are respectively aligned and positioned in the groove113 a at the top surface of the third base 113 and the groove 114 a atthe top surface of the fourth base 114. Conductive epoxy is deposited inthe groove 113 a and the groove 114 a for attaching the side metalportions 153 a and 153 b of the bridge-shaped metal sheet 150 to thethird base 113 and the fourth base 114 respectively. As such, thebridge-shaped metal clip 150 is precisely located in the groove 113 aand the groove 114 a. A bottom metal layer 133 at the back surface ofthe primary packaging structure 130 is connected to the bottom surfaceof the top metal portion 151 through the conductive material 140. InFIG. 4C, the plastic packaging material is deposited to form the plasticpackaging body 160 encapsulating the lead frame unit, the primarypackaging structure 130 and the bridge-shaped metal clip 150. The bottomsurfaces of the first base 111, the second base 112, the third base 113and the fourth base 114 of the lead frame unit are exposed from thebottom surface of the plastic packaging body 160, while the top surfaceof the top metal portion 151 can be selected whether to be exposed fromthe top surface of the plastic packaging body 160 or not. In FIG. 4C,the top metal portion 151 is covered by the plastic packaging body 160and the through hole 152 in the top metal portion 151 is filled withplastic packaging material.

FIGS. 5A-5B illustrate a structure of a power device 100B according toanother embodiment of the invention. The structure of power device 100Bis mostly similar as the structure of power device 100A excepting thestructure of the bridge-shaped metal clip 150. As shown in thesefigures, the top metal portion 151 does not include a through hole.Instead it includes pluralities of dimples 155 formed on the bottomsurface of the top metal portion 151. The dimples 155 extrude from thebottom surface of the top metal portion 151 and are located between thebottom metal layer 133 and the bottom surface of the top metal portion151 after the bridge-shaped metal clip 150 is mounted on the primarypackaging structure 130. With the dimples formed between the bottommetal layer 133 and the bottom surface of the top metal portion 151, thethickness of the conducting material 140 is uniform. As shown in FIG.5B, the top surface of the top metal portion 151 of the power device100B is not exposed from the plastic packaging body 160. Alternatively,the top surface of the top metal portion 151 can be exposed from the topsurface of the plastic packaging body 160 (not shown).

FIGS. 6A-6D illustrate a power device 100C of another embodiment of theinvention with the structure and the position of a second base of thelead frame unit different from that in the power devices 100A and 100B.FIGS. 6B and 6C are cross sectional schematic diagrams along the dottedlines AA and BB in FIG. 6A respectively. As shown in FIGS. 6A and 6B,the second base 212 includes a base extension 212 a and an external pin212 b connected to the base extension 212 a. The thickness of the baseextension 212 a is thinner than the thickness of the first base 111 andthus the base extension 212 a is encapsulated inside the plasticpackaging body 160. Only the bottom surface of the external pin 212 b isexposed from the bottom surface of the plastic packaging body 160.

As shown in FIG. 6A, the length of the fourth base 214 is shorter thanthe length of the third base 113 and the external pin 212 b is arrangedon the same side as the fourth base 214. Particularly the external pin212 b and a plurality of pins 214′ in the fourth base 214 are arrangedon the same straight line. The base extension 212 a extends under theprimary packaging structure 130 until the solder bump 132 a-1 on thefront surface of the primary packaging structure 130 superimposed on thebase extension 212 a. As such, the conducting material 120 a isdeposited for attaching the solder bumps 132 a-1 on the top surface ofthe base extension 212 a. As shown in FIGS. 6B-6C, the top surface ofthe base extension 212 a and the top surface of the first base 111 arearranged on the same plane substantially, so that the primary packagingstructure 130 is easily mounted on the first base 111 and the baseextension 212 a of the second base 212. The thickness of the baseextension 212 a is thinner than the thickness of the first base 111 sothat the extension base 212 a is encapsulated inside the plasticpackaging body 160 to avoid any negative effect on subsequent SMItechnology. The external pin 212 b and the fourth base 214 are arrangedon the same straight line, therefore, to avoid a short circuit betweenthe external pin 212 b and the bridge-shaped metal clip 150, as shown inFIG. 6D, the bridge-shaped metal clip 150 includes a shorter side metalportion 153′b for connecting to the fourth base 214 without connectingto the external pin 212 b of the second base 212. Particularly, thewidth D₁ of the side metal portion 153′b is smaller than the width D₂ ofthe top metal portion 151, while the width of the side metal sheet 153 ais the same as the width D₂ of the top metal portion 151. In the powerdevice 100C, the top surface of the top metal portion 151 is covered bythe plastic packaging body 160.

FIGS. 7A-7C illustrate a power device 100D of another embodiment of theinvention. The power device 100D is similar to the power device 100C,excepting that the top surface of the top metal portion 151 is exposedfrom the plastic packaging body 160. FIG. 7C is a top view of the powerdevice 100D showing the top metal portion 151 is exposed from theplastic packaging body 160, which is also used to improve the heatdissipation of the power device.

The above detailed descriptions are provided to illustrate specificembodiments of the present invention and are not intended to belimiting. Numerous modifications and variations within the scope of thepresent invention are possible. The present invention is defined by theappended claims.

The invention claimed is:
 1. A power device with bottom source electrodecomprising: a lead frame unit comprising a first base and a second baseseparated from each other, a third base and a fourth base located at twoopposite sides of the first base; a primary packaging structure attachedto the first base and the second base, the primary packaging structurecomprising a front surface and a plurality of solder bumps formedthereon, said solder bumps being further attached respectively to thefirst base and the second base; a bridge-shaped metal clip attached atopthe primary packaging structure, the bridge-shaped metal clip comprisesa top metal portion and side metal portions that are: connected to twoopposite sides of the top metal portion; and are bent downwards suchthat a first predetermined members of the side metal portions arepositioned and trapped into a groove formed on the top surface of thethird base and a second predetermined members of the side metal portionsare aligned and positioned and trapped into a groove formed on the topsurface of the fourth base; a bottom metal layer formed at a backsurface of the primary packaging structure and connected to a bottomsurface of the top metal portion through a conductive material; and aplastic packaging body encapsulating the lead frame unit, the primarypackaging structure and the bridge-shaped metal clip, wherein the bottomsurfaces of the first base, the third base and the fourth base are allexposed through the bottom surface of the plastic packaging body.
 2. Thepower device of the claim 1, wherein the primary packaging structurefurther comprising a semiconductor chip, a top plastic packaging layercovering both a front surface of the semiconductor chip and a bottommetal layer formed at a back surface of the semiconductor chip, whereinthe plurality of solder bumps are respectively attached to acorresponding number of metal pads formed on the front surface of thesemiconductor chip, and wherein the top plastic packaging layersurrounds side walls of the solder bumps so that the solder bumps areexposed through the top plastic packaging layer.
 3. The power device ofclaim 1, wherein the second base is located between the third base andthe fourth base.
 4. The power device of claim 3, wherein the primarypackaging structure further comprising a semiconductor chip, theplurality of solder bumps being respectively attached to a correspondingnumber of metal pads formed on the front surface of the semiconductorchip, the number of metal pads comprise at least a metal pad forming afirst electrode and a metal pad forming a second electrode of thesemiconductor chip, wherein those solder bumps connected to the metalpad forming the first electrode is connected to the top surface of thefirst base and those solder bumps connected to the metal pad forming thesecond electrode is connected to the top surface of the second base. 5.The power device of claim 3, wherein a bottom surface of the second baseis exposed through the bottom surface of the plastic packaging body. 6.The power device of claim 1 wherein the second base comprises a baseextension and an external pin connected to the base extension; the baseextension is thinner than the first base so that the base extension isencapsulated inside the plastic packaging body; the external pin islocated on the same line of the fourth base; and wherein the baseextension extends to the primary packaging structure until saidplurality of solder bumps overlap the base extension.
 7. The powerdevice of claim 6 wherein the primary packaging structure furthercomprising a semiconductor chip and said plurality of solder bumps arerespectively attached to a plurality of metal pads located on the frontsurface of the semiconductor chip, the plurality of metal padscomprising at least a metal pad forming the first electrode and a metalpad forming the second electrode of the semiconductor chip wherein thesolder bump connected to the metal pad forming the first electrode isattached to the top surface of the first base and the solder bumpconnected to the metal pad forming the second electrode overlaps and isattached to the base extension.
 8. The power device of claim 6 wherein abottom surface of the external pin is exposed through the bottom surfaceof the plastic packaging body.
 9. The power device of claim 6 wherein awidth of a side metal portion that is connected to the fourth base ismade smaller than the width of the top metal portion to prevent the sidemetal portion from connecting to the second base.
 10. The power deviceof claim 1 wherein one or more through holes are formed in the top metalportionand said through holes are configured to penetrate through thethickness of the top metal portion.
 11. The power device of claim 1wherein one or more clip grooves are formed from the bottom surface ofthe top metal portion to the inside of the top metal portion.
 12. Thepower device of claim 1 wherein a plurality of dimples are formed on thebottom surface of the top metal portion with said dimples extrudingbelow the bottom surface of the top metal portionand said dimples arelocated between the bottom metal layer and the bottom surface of the topmetal portion.
 13. The power device of claim 1 wherein a plurality ofrecessed portions are formed on the top surface of the top metal portionand further located at the corner between the top metal portion and theside metal portions.
 14. The power device of claim 2 wherein the bottommetal layer is formed on the back surface of the semiconductor chipforming a third electrode of the semiconductor chip, wherein thesemiconductor chip is a MOSFET (Metal Oxide Semiconductor Field EffectTransistor), in which the first electrode of the semiconductor chip is asource electrode, the second electrode of the semiconductor chip is agate electrode and the third electrode of the semiconductor chip is adrain electrode.
 15. The power device of claim 1 wherein the top surfaceof the top metal portion is covered by the plastic packaging body. 16.The power device of claim 1 wherein the top surface of the top metalportion is exposed through the plastic packaging body.